1. Field of the Invention
This invention relates to a steam aperture device in a mold for use in preparing a foamed article.
2. Description of the Prior Art
In order to prepare a foamed article of thermoplastic resin, a method has hitherto been used in which a mold is used having a cavity of a desired shape. Foamable beads of the thermoplastic resin are placed in the cavity, steam is blown into the cavity to foam and expand the beads until the beads are fused together in the cavity to form an integral foamed article, and the article is taken out from the cavity. In this instance, it is required that the mold be provided with a number of apertures, each of which has a small size so that the steam can pass through it, and softened and foamed resin cannot pass through it. The apertures are called "steam apertures".
Perforations or slits have hitherto been used for the steam apertures. The perforations used are small apertures, about 1 mm in diameter. The perforations have been located in a plate constituting a cavity wall either by perforating the plate or by embedding in the plate a number of small aperture members, each having a perforation. Likewise the slits have been made either by perforating the plate or by embedding in the plate a number of small members, each having a slit.
When a mold provided with the perforations or the slits is used for preparing a foamed article from the foamable beads, especially if the foamable beads are of great expanding ability, foamed particles have been liable to penetration into the perforations or the slits. The penetrating particles give rise to projections on the surface of the article when they remain on the article, and the penetrating particles give rise to conspicuous flaws on the article if they have been torn away from the article, thus they have decreased the quality of the product. Further, if the penetrating particles remain in the perforations or the slits, the steam is sometimes prevented from passing through the perforations or the slits, so that it becomes impossible to use the mold.